System and method for filtering network traffic

ABSTRACT

Protocol status information is used to perform traffic filtering by dropping messages that are not consistent with the protocol status information. In one embodiment, a method involves comparing message information and protocol status information. The message information is associated with a first message. The protocol status information is obtained in response to one or more second messages, which are conveyed according to a protocol used to assign network addresses to clients. The method also involves determining whether to discard the first message, based on an outcome of the comparison of the message information and the protocol status information. For example, it can be determined that the first message should be discarded, if the message information does not match the protocol status information.

RELATED APPLICATIONS

This application is a continuation-in-part of U.S. patent applicationSer. No. 10/654,388, entitled “System and Method for MaintainingProtocol Status Information in a Network Device,” filed Sep. 3, 2003 nowU.S. Pat. No. 7,343,485, and naming Dehua Huang, Adam J. Sweeney,Richard A. Johnson, and Silviu Dobrota as inventors, which is herebyincorporated by reference as if completely and fully set forth herein.

FIELD OF THE INVENTION

The present invention relates to networking and, more specifically, tonetwork devices that convey protocol messages implementing a networkprotocol between protocol clients and protocol servers and that useinformation in those protocol messages to restrict traffic between aprotocol client and a network.

BACKGROUND

Various network protocols are used to provide networking services todevices. For example, DHCP (Dynamic Host Configuration Protocol) allowsthe dynamic assignment of IP (Internet Protocol) addresses to hosts.Network protocols such as DHCP are often implemented using aclient-server model in which protocol clients request services fromprotocol servers, which in turn provide the requested services inaccordance with the network protocol. For example, in DHCP, a DHCPclient may request an IP address from a DHCP server. In response, theDHCP server sends a reply assigning an IP address to the requesting DHCPclient. Thereafter, the DHCP client uses the assigned IP address in amanner consistent with the protocol.

Protocol servers that provide networking services are vulnerable tovarious attacks. A denial of service attack may involve a usersimulating a large number of protocol clients in order to exhaust theserver's available pool of resources. A denial of service attack mayalso involve a user sending a high rate of client requests to theprotocol server in order to inhibit the ability of the protocol serverto respond to legitimate client requests.

Similarly, some users may falsify or steal protocol information in orderto perform undesirable activity via a network. For example, a user maysimulate a protocol server and send fake protocol responses. Theseresponses may corrupt legitimate network operation. Similarly, a usermay snoop a response sent by a legitimate protocol server to a protocolclient and use the network services being provided in those responses.For example, if a DHCP response assigning an IP address to a client isintercepted, the intercepting user may deceptively pretend to be theclient by using the IP address obtained from the DHCP response. As theseexamples show, network protocols may experience various securityproblems that result from misuse of the network protocol and/or attackson protocol servers.

SUMMARY

Various embodiments of systems and methods for maintaining protocolstatus information that includes protocol information generated by aprotocol server and information indicative of how messages are conveyedvia a network are disclosed. This protocol status information can thenbe used to perform traffic filtering by dropping messages that are notconsistent with the protocol status information. Such systems andmethods may be used to inhibit the misuse of one or more networkprotocols and/or attacks on protocol servers.

In some embodiments, a method involves comparing message information andprotocol status information. The message information is associated witha first message. The protocol status information is obtained in responseto one or more second messages, which are conveyed according to aprotocol used to assign network addresses to clients. The method alsoinvolves determining whether to discard the first message, based on anoutcome of the comparison of the message information and the protocolstatus information. For example, it can be determined that the firstmessage should be discarded, if the message information does not matchthe protocol status information.

The protocol status information can be generated in response to the oneor more second messages and stored in a binding table entry. In oneembodiment, each binding table entry includes information thatidentifies an Internet Protocol (IP) address of a client, a Media AccessControl (MAC) address of the client, and an interface coupled to theclient.

At least some of the comparison of the message information to theprotocol status information can be performed in response to an accesscontrol rule. For example, the access control rule can require that amessage received via the interface be dropped unless a IP source addressof the message matches the IP address of the client identified in theprotocol status information. The access control rule can also require amessage received via the interface to be dropped unless a MAC sourceaddress of message matches the MAC address of the client. Informationencoding the access control rule can be updated each time that acorresponding binding table entry is updated.

A second access control rule can specify that a Dynamic HostConfiguration Protocol (DHCP) message received via the interface beprocessed by a snooping agent. The snooping agent is configured toupdate protocol status information in the binding table in response toDHCP messages. The second access control rule can also require that aDHCP message received via any one of several interfaces (or even allinterfaces belonging to a particular VLAN or network device) beprocessed by the snooping agent. Thus, a single access control rule(e.g., stored in a single entry of an access control list) can be usedto detect DHCP messages received via multiple interfaces and to causethose detected DHCP messages to be processed by the snooping agent.

In one embodiment (e.g., an embodiment supporting private VLANs), abinding table entry, which stores protocol status information, isupdated in response to detecting a first protocol message, which isbeing conveyed from a DHCP server in a first VLAN (e.g., a primaryVLAN). This protocol status information can be used when determiningwhether to discard a non-protocol message being conveyed in a secondVLAN (e.g., a secondary VLAN).

In some embodiments, a network device includes a binding table and afiltering module coupled to the binding table. The binding table isconfigured to store protocol status information. The protocol statusinformation is obtained in response to one or more first messages, whichare conveyed according to a protocol used to assign network addresses toclients. The filtering module is configured to compare messageinformation with the protocol status information. The messageinformation is associated with a second message. The filtering module isconfigured to determine whether to discard the second message, inresponse to comparing the message information with the protocol statusinformation.

The foregoing is a summary and thus contains, by necessity,simplifications, generalizations and omissions of detail; consequently,those skilled in the art will appreciate that the summary isillustrative only and is not intended to be in any way limiting. Theoperations disclosed herein may be implemented in a number of ways, andsuch changes and modifications may be made without departing from thisinvention and its broader aspects. Other aspects of the presentinvention, as defined solely by the claims, will become apparent in thenon-limiting detailed description set forth below.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete understanding of the present invention may be acquiredby referring to the following description and the accompanying drawings,in which like reference numbers indicate like features.

FIG. 1 is a block diagram of network coupling protocol clients to aprotocol server, according to one embodiment.

FIG. 2A shows a block diagram of a switch configured to snoop protocolmessages being conveyed via a network and to update protocol statusinformation in response to the snooped protocol messages, according toone embodiment.

FIG. 2B shows another block diagram of a switch configured to snoopprotocol messages being conveyed via a network and to update protocolstatus information in response to the snooped protocol messages,according to one embodiment.

FIG. 2C is yet another block diagram illustrating an example of protocolstatus information maintained by a network device, according to oneembodiment.

FIG. 3 is a flowchart illustrating one embodiment of a method ofsnooping protocol messages in order to update protocol statusinformation and to perform security actions dependent on the protocolstatus information.

FIG. 4A is a flowchart illustrating various security actions that can beperformed dependent upon protocol status information, according to oneembodiment.

FIG. 4B is a flowchart illustrating additional details of how protocolstatus information is used to verify protocol messages in the embodimentof FIG. 4A.

FIG. 5 is a flowchart illustrating how protocol status information isobtained by intercepting a protocol request from a protocol client and aprotocol reply from a protocol server and how this protocol statusinformation is then used to verify other protocol messages, according toone embodiment.

FIG. 6 illustrates an example of a switch that is configured to performnetwork traffic filtering, according to one embodiment of the presentinvention.

FIG. 6A illustrates another example of a switch, according to anotherembodiment of the present invention.

FIG. 7 shows another example of a switch that is configured to performnetwork traffic filtering, according to one embodiment of the presentinvention.

FIG. 8 is a flowchart of one embodiment of a method of performingnetwork traffic filtering.

FIG. 9 illustrates a switch that supports private VLANs and isconfigured to perform network traffic filtering, according to oneembodiment of the present invention.

FIG. 10 is a block diagram of a switch illustrating how a snooping agentand access control rule update module can be implemented in softwareexecuting on the switch, according to one embodiment.

While the invention is susceptible to various modifications andalternative forms, specific embodiments of the invention are provided asexamples in the drawings and detailed description. It should beunderstood that the drawings and detailed description are not intendedto limit the invention to the particular form disclosed. Instead, theintention is to cover all modifications, equivalents and alternativesfalling within the spirit and scope of the invention as defined by theappended claims.

DETAILED DESCRIPTION

The following sets forth a detailed description of at least the bestcontemplated mode for implementing the devices and/or processesdescribed herein. The description is intended to be illustrative andshould not be taken to be limiting.

Protocol Snooping

FIG. 1 is a block diagram of network 10 coupling protocol clients12A-12C (collectively, protocol clients 12) to protocol server 16,according to one embodiment. Network 10 includes several networkdevices, including switches 20A-20B (collectively, switches 20). Notethat other embodiments may include significantly more network devices(and network devices of different types, such as routers) in network 10.Each switch 20 in network 10 may be configured to handle messages fromother network devices and from various computing devices coupled by thenetwork 10. At least some of these messages may be protocol messagesbeing sent between protocol clients 12 and protocol server 16.

In a communications network such as network 10, switching devices suchas switches 20 operate by receiving data at one of a set of inputinterfaces and forwarding the data on to one or more of a set of outputinterfaces. Users typically require that such switching devices operateas quickly as possible in order to maintain a high throughput. Switchesare typically data-link layer devices that enable multiple physicalnetwork (e.g., local area network (LAN) or wide area network (WAN))segments to be interconnected into a single larger network. Switches 20are, in some embodiments, implemented as OSI (Open SystemsInterconnection) layer 2 devices. Switches 20 can include logic and/orsoftware that provides for the forwarding of messages based on themessages' destination information.

The data received and forwarded by switches 20 is logically grouped intoone or more messages. Throughout this disclosure, the term “message” isused to refer to a logical grouping of information sent as a data unitover a transmission medium. Messages may include header and/or trailerinformation that surrounds user data contained in the data unit. Forpurposes of this disclosure, a “message” may include a cell, datagram,frame, packet, segment, or any other logical group of information.

Switches 20 include various interfaces to protocol clients 12 andprotocol server 16. These interfaces may include physical interfaces(e.g., on a line card internal to a switch) and/or logical interfaces(also referred to as virtual interfaces). For example, a physicalinterface that receives messages for several VLANs (Virtual Local AreaNetworks) can include several logical interfaces, one for each VLAN.Alternatively, a logical interface can be an interface that is locatedon a physically separate intermediate network device, coupled betweenthe switch and a group of protocol clients, that passes messages sent bythe protocol clients to the switch without making forwarding decisions(this type of interface is also described as a remote interface).Furthermore, the interfaces may be organized in various ways. Forexample, in some embodiments, interfaces are organized hierarchically.In one such embodiment, physical interfaces within a switch reside atthe top level of the hierarchy. A physical interface to devices inseveral different VLANs can include several VLAN-specific logicalinterfaces that are organized beneath the physical interface in theswitch's interface hierarchy.

In some embodiments, protocol clients 12 and protocol server 16 eachinclude one or more of various types of computing devices. For example,protocol clients 12 and/or protocol server 16 can each be a personalcomputer, a workstation, an Internet server, a network appliance, ahandheld computing device such as a cell phone or PDA (Personal DataAssistant), or any other type of computing device. Protocol clients 12and protocol server 16 can also be implemented in software processesexecuting on such computing devices.

Protocol clients 12 and protocol server 16 can each be directly orindirectly connected to one of switches 20. For example, in someembodiments, protocol clients 12A and 12B are coupled directly to switch20A (e.g., switch 20A is included in a wiring closet on the same site asprotocol clients 12A and 12B). In other embodiments, protocol clients12A and 12B are indirectly coupled to switch 20A through one or moreintermediate network devices that send messages received from protocolclients 12A and 12B to switch 20A. Similarly, switches 20A and 20B canbe directly or indirectly coupled to each other. For example, one ormore routers (as well as one or more other switches or other networkdevices) can be interposed between switch 20A and switch 20B.

Protocol server 16 provides network services to protocol clientsaccording to a network protocol. For example, in one embodiment,protocol server 16 is configured to implement a DNS (Domain Name System)server. A protocol client 12 can request an IP (Internet Protocol)address that corresponds to a particular domain name from DNS protocolserver 16. DNS protocol server 16 responds by sending the protocolclient 12 a message that includes the requested IP address according tothe DNS protocol. Protocol client 12 can then use the IP addressassigned by the DNS protocol server 16 to communicate with other clientsvia network 10.

In another embodiment, protocol server 16 is configured to implement aDHCP (Dynamic Host Configuration Protocol) server. A protocol client 12may send a message requesting an IP address from protocol server 16. Inresponse to the request, protocol server 16 sends a message assigning anIP address to the protocol client 12.

In other embodiments, many additional network service protocols can beimplemented in addition to and/or instead of DNS and DHCP. For example,in another embodiment, protocol server 16 is a Reverse ARP (AddressResolution Protocol) server.

Each switch 20 (and/or router or other network device in network 10)includes a snooping agent 50. As shown in FIG. 1, switch 20A includessnooping agent 50A and switch 20B includes snooping agent 50B.

Each snooping agent 50 is configured to intercept protocol messagesbeing sent between protocol clients 12 and protocol server 16 vianetwork 10. In response to the intercepted protocol messages, snoopingagent 50 updates protocol status information locally to a network device(e.g., switch 20 or a router) in which snooping agent 50 is implemented.Thus, each snooping agent 50 is an example of a means for updatingprotocol status information in response to intercepting a protocolmessage being conveyed between a protocol client and a protocol server.This protocol status information includes network traffic informationindicative of how messages (both protocol messages and non-protocolmessages) are conveyed to network 10 by each of protocol clients 12. Forexample, snooping agent 50A can store information indicating which ofthe physical and/or logical interfaces included in switch 20A received aprotocol message from client 12B requesting a network service. Theprotocol status information may also include information included in theprotocol messages themselves. For example, snooping agent 50A can storeinformation, such as an IP address, included in protocol server 16'sreply to the protocol request from client 12B. Thus, the protocol statusinformation can also include protocol information generated by protocolserver 16. Each snooping agent 50 can then use its protocol statusinformation to determine whether to perform various security actionssuch as dropping messages.

FIG. 2A shows a block diagram of a switch 20 that snoops protocolmessages being conveyed via a network and updates protocol statusinformation in response to the snooped protocol messages. As shown inFIG. 2A, switch 20 includes a protocol message detector 24, a snoopingagent 50, and a storage unit that stores protocol status information 60.Protocol status information 60 can be stored in various types of memory,including ROM (Read Only Memory), RAM (Random Access Memory), MEMS(Micro Electro-Mechanical Systems) memory, Flash memory, and the like.

As shown in FIG. 2A, switch 20 may receive an input message stream 200via various logical and/or physical interfaces. The input message stream200 may include various protocol messages 202 being conveyed betweenprotocol server 16 and protocol clients 12 of FIG. 1. Additionalmessages 204 (e.g., data messages or messages implementing otherprotocols that are not intercepted by snooping agent 50) being conveyedvia network 10 of FIG. 1 may also be included in the input messagestream 200. Once the protocol messages 202 identified by protocolmessage detector 24 are processed by snooping agent 50, snooped protocolmessages 202A (which may include modified versions of the protocolmessages 202 input to snooping agent 500 and/or fewer messages thaninput protocol messages 202) are output with non-protocol messages 204as part of output message stream 220 to other network devices, protocolclients, and/or protocol servers).

FIG. 2B illustrates how switch 20 may include several interfaces 22A-22n (collectively, interfaces 22) that each interface to one or more othernetwork devices, protocol clients, and/or protocol servers. Interfaces22 can each be a logical or a physical interface. For example,interfaces 22A-22B can be included in the same physical interface, butinterface 22A may handle messages being conveyed in one VLAN whileinterface 22B handles messages being conveyed in another VLAN. As notedabove, in some embodiments, interfaces 22 can be organizedhierarchically.

Returning to FIG. 2A, protocol message detector 24 is configured toidentify protocol messages 202 that are received in input message stream200. In one embodiment, protocol message detector 24 can be implementedusing a CAM (Content Addressable Memory) and/or ACL (Access ControlList). For example, information from each message can be input into aCAM, which responds by outputting a corresponding index, which is thenused to lookup information in an ACL. If the message is a protocolmessage, the indexed information in the ACL may indicate that theprotocol message should be handled by snooping agent 50. Alternatively,protocol message detector 24 can be implemented as a software processthat analyzes each message in order to identify which messages areprotocol messages. In other embodiments, other types of software and/orhardware can be used to implement protocol message detector 24.

In some embodiments, information in each message that identifies asource and/or destination software port is used by protocol messagedetector 24 to identify protocol messages. For example, if protocolmessages requesting network services provided by the DNS protocol areaddressed to server port 53, protocol message detector 24 can readilyidentify protocol messages to server port 53 as DNS protocol messages.

Whenever protocol message detector 24 identifies a protocol message(e.g., if the protocol being snooped is DHCP, protocol message detector24 can identify DHCP messages in the message stream based on thesoftware port to or from which the messages are addressed, as describedabove), the protocol message detector 24 causes the identified protocolmessage to be forwarded to snooping agent 50. For example, in oneembodiment, protocol message detector 24 generates an interrupt, causinga software process implementing all or part of snooping agent 50 to beinitiated to handle the identified protocol message. Snooping agent 50can include a queue (not shown) to temporarily buffer protocol messagesidentified by protocol message detector 24.

In some embodiments, protocol message snooping is limited to aparticular VLAN (Virtual Local Area Network) or performed differentlydepending on the VLAN in which the protocol message is being conveyed.In these situations, protocol message detector 24 also takes VLAN intoaccount when identifying protocol messages within the input messagestream. For example, if protocol message snooping is enabled for oneVLAN but not for another, protocol message detector 24 only identifiesprotocol messages being conveyed in the VLAN for which snooping isenabled.

In response to receiving protocol messages detected by protocol messagedetector 24, snooping agent 50 performs various functions on and/or inresponse to the protocol messages. In the embodiment of FIG. 2A,snooping agent 50 includes filtering unit 51, rate limiting unit 53,binding unit 55, relay information unit 57, and unicast unit 59. Inother embodiments, snooping agent 50 may include fewer or additionalfunctional units. Snooping agent 50 can be implemented in hardware,software, or a combination of the two.

Filtering unit 51 filters protocol messages that are not consistent withprotocol status information 60 from the message stream (e.g., bydropping those messages). In one embodiment, filtering unit 51 canverify a protocol message by comparing information included in aparticular protocol message with information stored in protocol statusinformation 60. If the information does not match, filtering unit 51drops the message. For example, filtering unit 51 can handle a protocolmessage that includes information indicating that the protocol messageis a reply from a protocol server to a protocol client. Filtering unit51 accesses information included in protocol status information 60 todetermine whether the protocol client currently has an outstandingprotocol request. If there is no corresponding outstanding request,filtering unit 51 drops the reply. In some embodiments, performing thistype of filtering makes it more difficult for a protocol server to beimitated by another device coupled to the network.

Similarly, filtering unit 51 can determine whether a protocol message(or non-protocol message) that includes information indicating that themessage was sent by a particular client or server was actually sent bythat client or server. This determination is based on information inprotocol status information 60 indicating what network services (e.g.,IP addresses) have been provided to particular clients and how protocolclients convey messages to the network. For example, protocol statusinformation 60 can include network traffic information associating eachclient and each server with a particular interface 22 of switch 20. Thisinformation is typically obtained by snooping agent 50 the first time aprotocol message from each client and each server is received into theswitch 20 (in some embodiments, such information may also be obtained inresponse to non-protocol messages). This information is then maintainedin protocol status information 60.

If a subsequent protocol or non-protocol message purporting to be from aparticular client or server is received via a different interface thanthe interface identified in protocol status information 60 as beingassociated with that client or server, filtering unit 51 drops themessage. This type of filtering interferes with the ability of a devicecoupled to the network to imitate a legitimate protocol client orserver. Types of filtering such as this can be used to verifynon-protocol messages being sent by a protocol client to a device otherthan a protocol server.

Rate limiting unit 53 drops protocol messages if a rate limit forprotocol messages is exceeded. Information indicating the rate limit isincluded in protocol status information 60 in some embodiments.Depending on the embodiment, rate limiting may be performed on a perinterface and/or per type (e.g., client-generated or server-generated)of protocol message basis. For example, client-generated protocolmessages can be rate limited, while server-generated protocol messagesare not rate limited. Alternatively, an independent rate limit can beassociated with each interface (or group of interfaces). The ratelimiting unit 53 applies the associated rate limit to protocol messagesbeing communicated via a particular interface. If the rate limit at aparticular interface (or group of interfaces) is exceeded, rate limitingunit 53 can disable that interface (or group of interfaces).

In one embodiment, rate limiting unit 53 implements a rate limitingalgorithm based on the time at which a protocol message is received andthe current value of a counter indicative of how many protocol messageshave already been received during a given time interval. The counter canbe associated with a particular interface (e.g., each interface 22 ofFIG. 2B can have an associated counter, in one embodiment) or with agroup of interfaces. The counter is reset periodically or reset based oncertain conditions. Each time a new protocol message is received via anassociated interface, rate limiting unit 53 increments the counter. Foreach protocol message received via the associated interface, ratelimiting unit 53 compares the value in the counter to the rate limit forthe associated interface. If the comparison indicates that the ratelimit is exceeded, rate limiting unit 53 drops the protocol message,disables the associated interface, and/or logs an error message.

Binding unit 55 stores information obtained from intercepted protocolmessages as part of protocol status information 60. For example, in anembodiment in which snooping agent 50 snoops DHCP messages, binding unit55 can store an IP address assigned to a DHCP client by a DHCP server aspart of protocol status information 60. Binding unit 55 can also storeadditional information identifying the DHCP client with that IP address.For example, binding unit 55 can store the DHCP client's MAC (MediaAccess Control) address and information identifying the VLAN in whichthat DHCP client is included. Binding unit 55 can also store informationindicating how messages are communicated to that DHCP client. Forexample, binding unit 55 can store information identifying whichinterface 22 is coupled to receive messages from that DHCP client.Binding unit 55 also modifies (e.g., by changing, supplementing, and/ordeleting) existing protocol status information 60 in response tosubsequent protocol messages.

The integrity of protocol status information 60 affects how wellfiltering unit 51 identifies protocol messages to filter from themessage stream. Accordingly, binding unit 55 can be configured to onlymodify protocol status information 60 in response to a snooped protocolmessage if that message has already been verified as being consistentwith protocol status information 60 by filtering unit 51. This way,binding unit 55 will not modify protocol status information 60 inresponse to a protocol message that is filtered by filtering unit 51.

In some embodiments, binding unit 55 updates protocol status information60 in response to protocol messages that are received via an interfaceto protocol clients and/or servers but does not update protocol statusinformation 60 in response to protocol messages received via aninterface to another network device that includes its own snooping agent50. Stated another way, if switch 20 is the first switch (or othernetwork device that includes snooping agent 50) in network 10 to handlemessages sent by a particular protocol client, binding unit 55 updatesprotocol status information 60 in response to messages sent to and/orfrom that protocol client. If switch 20 is not the first switch innetwork 10 to handle messages sent by that protocol client, binding unit55 will not update protocol status information 60 in response tomessages to and from that protocol client. In this way, each switch innetwork 10 may be responsible for maintaining protocol statusinformation 60 for a subset of the total group of protocol clientsand/or servers coupled by the network. For example, looking at FIG. 1,switch 20A is responsible for updating its protocol status informationin response to protocol messages to and from protocol clients 12A and12B, while switch 20B is responsible for updating its protocol statusinformation in response to protocol messages to and from protocol client12C and protocol server 16.

Information indicating which interfaces 22 are coupled to protocolclients and/or servers and which interfaces 22 are coupled to othernetwork devices may be programmed into switch 20 in some embodiments.For example, configuration information indicating which interfaces 22are coupled to other network devices that include snooping agents 50 andwhich interfaces 22 are coupled to protocol clients may be loaded into amemory device included in switch 20 by a network administrator.Alternatively, each switch can learn which of its interfaces are coupledto protocol clients and/or servers, as opposed to being coupled to othernetwork devices, by snooping protocol and/or non-protocol messages andupdating protocol status information 60 to include informationindicative of network traffic in response to the snooped messages, asdescribed above.

Binding unit 55 may store information, generated by a protocol server,indicating how assigned network services may be used by a protocolclient to which those services are assigned (such information may bestored in binding table 70, as described below with respect to FIG. 2C).For example, in certain protocols such as DHCP, assigned networkservices can be associated with lease times indicative of how long aclient is allowed to continue using its assigned network services. Inembodiments in which protocol messages for such a protocol are beingsnooped, binding unit 55 establishes a lease timer (e.g., as a value orset of values in protocol status information 60 that is incremented ordecremented periodically). Similar information indicative of how aprotocol server has indicated that a network service may be used by aclient to which that network service is assigned may also be stored inprotocol status information 60. Filtering unit 51 can use thisinformation to determine whether a client is using its assigned networkservices consistently with the protocol server's limitations on thosenetwork services. If a protocol client that sends a protocol message isnot using an assigned network service consistently with the informationstored by binding unit 55, filtering unit 51 may drop the protocolmessage.

Relay information unit 57 verifies and/or modifies relay informationincluded in protocol messages. Relay information is information that isbe added to protocol messages by network devices (e.g., routers andswitches) and used by various network devices to verify certain protocolmessage behavior. Relay information can be removed from protocolmessages before those protocol messages are conveyed to protocolclients. One example of such relay information is provided in RFC 3046,which describes relay information that can be added to DHCP messages.Relay information unit 57 (and/or filtering unit 51) verifies protocolmessages based on relay information included within those protocolmessages. Relay information unit 57 can also remove relay informationfrom a protocol message before that protocol message is sent to aprotocol client. In one embodiment, relay information unit 57 adds relayinformation to the protocol message if switch 20 is the first networkdevice with a snooping agent in network 10 to handle the protocolmessage. If switch 20 is not the first network device with a snoopingagent in network 10 to handle the protocol message, relay informationunit 57 verifies any preexisting relay information.

Unicast unit 59 detects protocol messages being sent from a protocolserver to a protocol client and causes the detected messages to beunicast to the destination protocol client instead of being broadcast ormulticast to multiple recipients. By unicasting the protocol messages,the ability of a device other than the destination client to snoop theinformation being provided to the client by the protocol server isinhibited. For example, if a DHCP ACK message is being sent, unicastingthe message to the destination client makes it more difficult foranother device (which might have received the DHCP ACK if the DHCP ACKhad been multicast) to obtain the IP address being assigned to thatclient. This in turn makes it more difficult for the non-destinationclient to imitate the destination client device on the network.

In one embodiment, snooping agent 50 performs its various functions inthe following order: rate limiting, filtering, binding, add/modify relayinformation, and unicasting. However, this order is merely an example.Other embodiments may order these functions differently.

In some embodiments, switch 20 snoops protocol messages being sent toimplement several different protocols. For example, switch 20 can snoopDHCP, DNS, and Reverse ARP messages, generate protocol statusinformation in response to the snooped messages, and perform securityactions based on the protocol status information. In some suchembodiments, switch 20 includes multiple protocol message detectors 24(e.g., one protocol message detector 24 for each protocol beingsnooped), a single protocol message detector 24 configured to identifyprotocol messages for multiple different protocols, or a combination ofsuch protocol detectors. Similarly, switch 20 can include multipleprotocol-specific snooping agents 50 or, alternatively, one snoopingagent 50 can maintain protocol status information for multiple differentprotocols. In some embodiments, protocol status information for eachprotocol is maintained independently. In other embodiments, protocolstatus information for the different protocols is collectivelymaintained and used to perform security actions. For example,information obtained by snooping DHCP messages can be used to filter DNSmessages.

It is noted that snooping agent 50 and/or protocol message detector 24can be implemented differently in different types of network devices.For example, protocol message detector 24 can be implemented in hardwarewithin a switch 20 while being implemented in software within a router.

FIG. 2C is a block diagram illustrating an example of protocol statusinformation that can be maintained by a network device such as switch20. In this embodiment, snooping agent 50 intercepts DHCP messages andupdates binding table 70 in protocol status information 60 in responseto the snooped DHCP messages. Binding table 70 is similar to a bindingtable typically maintained by a DHCP server. Binding table 70 includesone or more entries 62A-62 n (collectively, entries 62) that eachcorrespond to a particular protocol client. As represented by entry 62B,an entry 62 includes client IP address 71, client MAC address 73,interface ID 75 (e.g., identifying the logical and/or physical interface22 via which messages sent by the client are received into the switchthat includes binding table 70), VLAN (Virtual Local Area Network) ID 77(identifying a VLAN in which the client to which entry 62 correspondssends protocol messages), and lease time 79. The information illustratedas being included in binding table 70 is merely an example. In otherembodiments, different information can be included in a binding table inaddition to and/or instead of the information shown in FIG. 2C.

Client IP address 71 may have been generated by protocol server 16 (inthis example, a DHCP server) of FIG. 1. Snooping agent 50 obtains clientIP address 71 from a protocol message being sent from protocol server 16to a particular protocol client 12 and stores client IP address 71 in abinding table entry corresponding to that protocol client. Lease time 79is a value representing the length of time that client IP address 71 maybe used by the client to which that IP address is assigned. Snoopingagent 50 also obtains this information from the DHCP server replyassigning the IP address to the client.

Snooping agent 50 can obtain other information for storage in bindingtable 70 from protocol messages generated by clients 12. For example,snooping agent 50 may allocate a binding table entry 62 to a particularclient in response to intercepting a DHCP request from that clientrequesting an IP address from a DHCP server. Snooping agent 50 canobtain client MAC address 73, interface ID 75, and/or VLAN ID 77 fromthe DHCP request and store this information in the allocated bindingtable entry.

In one embodiment in which DHCP messages are being intercepted, snoopingagent 50 responds to a DHCP ACK message (sent from a DHCP server to aDHCP client to assign an IP address to that client) by allocating anentry 62 in binding table 70 to the DHCP client, if such an entry doesnot already exist. In response to a DHCP NAK message (sent from a DHCPserver to a DHCP client to deny assigning an IP address to that client),snooping agent 50 deletes an existing binding table entry (if any)corresponding to that client. A DHCP NAK message is sent when a clientattempts to reuse an IP address and the DHCP server determines that theIP address is not applicable to the client (e.g., because the client hasmoved to a new subnet). Similarly, in response to a DHCP RELEASE message(sent from a client to a DHCP server to relinquish an IP addressassigned to that client), snooping agent 50 deletes an existing bindingtable entry corresponding to that client. If a DHCP DECLINE message(sent from a client to a DHCP server in response to the clientdiscovering that an IP address is already assigned to another device) isreceived, snooping agent 50 deletes an existing binding table entry (ifany) corresponding to that client.

Generally, snooping agent 50 stores various information, such as an IPaddress, identifying a network service provided by a protocol server toa protocol client in a binding table entry associated with a particularclient. Snooping agent 50 can also store other information identifyingthat client, such as a client MAC address and an interface ID, in thebinding table entry. Snooping agent 50 may use this information toverify subsequent protocol messages. For example, snooping agent 50 canverify the sender of a particular protocol message by comparing thesource IP address, MAC address, and interface ID in a particular messagewith the information stored in binding table 70. If no entry 62 inbinding table 70 includes the same combination of source IP address, MACaddress, and interface ID, snooping agent 50 determines that theprotocol message was not sent by the client identified as the senderand, in response to that determination, drop the protocol message.

A binding table 70 can also be used to store information, such as leasetime 79, indicating how network services provided by a particularprotocol are to be used. Snooping agent 50 may use this information toverify that network services (e.g., IP addresses) are being used in amanner that is consistent with the protocol. For example, using leasetime information 79, snooping agent 50 can examine intercepted protocolmessages to determine whether a client is attempting to use an IPaddress for longer than the client is allowed. If a protocol violationis detected (e.g., if a message sent by a client having a particular IPaddress is received subsequent to the expiration of the lease timeassociated with that IP address in the binding table) in a particularmessage, snooping agent 50 drops that message.

In some embodiments, binding table 70 is implemented as a hash table. Toaccess a binding table entry associated with a particular client,information such as the client's MAC address and VLAN ID is used as ahash key.

FIG. 3 is a flowchart illustrating one embodiment of a method ofsnooping protocol messages in order to update protocol statusinformation and to perform security actions dependent on the protocolstatus information. At 300, one or more protocol messages beingcommunicated via a network are intercepted by a network device. Theprotocol messages are included in a stream of messages input to thenetwork device. In some embodiments, the network device may detectprotocol messages based on a server port indicated in the protocolmessages.

In response to the intercepted protocol messages, the network deviceupdates protocol status information, as indicated at 302. The protocolstatus information includes both information included in the protocolmessages and information indicative of how the protocol messages areconveyed from one or more protocol clients to the network. For example,in response to receiving a protocol message sent by a protocol client torequest a network service, the network device can store informationindicating that the protocol request was received via a particularlogical and/or physical interface of the network device. The networkdevice can store information indicating that the protocol client has anoutstanding protocol request. The network device can also storeinformation included in the protocol request that identifies the clientdevice (e.g., a VLAN ID and/or MAC address). Similarly, in response to aprotocol message sent by a protocol server to provide a network serviceto a client, the network device stores information identifying thenetwork service (e.g., an IP address), the client to which that serviceis being provided, and/or information (e.g., a lease time) indicatinghow the client may use the service. The protocol status information canalso include information indicative of the rate at which protocolmessages are being received from a particular client or group of clientsor via a particular interface or group of interfaces.

The network device then uses the protocol status information updated at302 to determine whether to perform security actions for subsequentlyreceived messages. As indicated at 304, the network device performs asecurity action for a subsequent protocol message dependent on theprotocol status information and the type (e.g., client request, serverresponse, and the like) of the subsequent protocol message. Securityactions include dropping the protocol message if it includes informationthat is inconsistent with the protocol status information, updatinginformation included in the protocol message, and/or updating an errorlog. Note that the network device can also update the protocol statusinformation in response to the subsequently received protocol message.

In some embodiments, the network device also uses the protocol statusinformation to determine whether to perform security actions in responseto non-protocol messages, as indicated at 306. For example, a networkdevice may receive, via one of the network device's interfaces, amessage containing information indicating that it was sent by a clienthaving a certain IP address, assigned using the network protocol. Thenetwork device can verify whether the protocol server assigned that IPaddress to a client that sends messages to the network via thatinterface of the network device. If the protocol status informationindicates that the client assigned that IP address sends messages to thenetwork via another interface of the network device, or that no clienthas been assigned that IP address, the network device performs asecurity action such as dropping the message and/or updating an errorlog. If the protocol status information instead indicates that theclient assigned that IP address does convey messages to the network viathat interface of the network device, the network device may allow themessage to continue its progress through the network.

FIG. 4A is a flowchart illustrating various security actions that can beperformed dependent upon protocol status information. In thisembodiment, a network device receives a protocol message via aparticular logical and/or physical interface, as indicated at 400. Basedon protocol status information maintained by that network device, thenetwork device determines whether the interface that received theprotocol message is allowed to issue that type of protocol message, asshown at 402. If the interface is not allowed to issue that type ofprotocol message (e.g., if the interface is not coupled to receivemessages from a protocol server, and if the message is of a type that isonly allowed to be sent by a protocol server), the network device dropsthe message, as shown at 420. Otherwise, the network device proceeds tofurther verify the protocol message at 404.

As indicated at 404, the network device verifies whether a rate limitfor protocol messages has been exceeded. If so, the network device dropsthe protocol message, as shown at 420. The rate of protocol messages canbe limited on a per interface (or group of interfaces) and/or a perclient (or group of clients) basis in some embodiments.

At 406, the network device verifies whether protocol relay information(if any) included in the protocol message is correct. For example, ifthe protocol message is a DHCP message, the network device can check tosee if another network device has inserted relay information into theprotocol message. If so, the network device determines whether theprotocol relay information is correct, based on information indicativeof the interface via which the message was received.

If the protocol relay information (if any) in the protocol message iscorrect, the network device can modify or add protocol relay informationto the protocol message, as indicated at 408. For example, if noprotocol relay information is included in the protocol message (e.g.,because this network device is the first network device allowed to addrelay information to handle the protocol message), the network deviceadds protocol relay information to the protocol message. Alternatively,if the network device is outputting the protocol message to a client orserver (as opposed to another network device), the network deviceremoves any protocol relay information already included in the message.

If the protocol message is a type of message (e.g., a protocol replysuch as a DHCP ACK or DHCP NAK) sent by a protocol server, as determinedat 412, the network device accesses the protocol status information todetermine whether the protocol message corresponds to an outstandingprotocol request from a client, as shown at 416. If the protocol messagedoes not correspond to an outstanding request, the network device dropsthe protocol message, as shown at 420. Otherwise, the network deviceunicasts the protocol message to the destination client so that no otherclients receive the protocol message, as indicated at 418. If theprotocol message is not a type of message sent by a protocol server, thenetwork device transmits the protocol message normally (e.g., if theprotocol message is identified as a message that should be flooded ormulticast, the network device will appropriately flood or multicast theprotocol message), as shown at 414.

FIG. 4B is a flowchart illustrating additional details of how protocolstatus information is used to verify that a particular interface isallowed to issue a particular type of protocol messages in theembodiment of FIG. 4A. At 430, the network device determines whether theprotocol message is of a type that must be sent by a protocol server.For example, if the protocol message is a DHCP message, the networkdevice identifies messages of types DHCP OFFER, DHCP ACK, and DHCP NAKas messages that only a DHCP server is allowed to send. If the protocolmessage is of a type that is not required to be sent by a protocolserver, the network device determines that the interface is allowed toissue this type of protocol message, as shown at 434. Note that in someembodiments, if the protocol message is of a type that may only be sentby a protocol client, the network device additionally verifies whetherthere are any protocol clients coupled to send messages via thatinterface before reaching the determination at 434.

If the protocol message is of a type that can only be sent by a protocolserver, the network device accesses protocol status information todetermine whether the interface via which the protocol message wasreceived into the network device is coupled to receive messages from aprotocol server, as shown at 432. If the protocol status informationindicates that a protocol server is coupled to that interface, then thenetwork device determines that the interface is allowed to issue thattype of protocol message. Otherwise, the network device determines thatthe interface is not allowed to issue that type of protocol message, asindicated at 436. In response, the protocol message is dropped, as shownat 420 of FIG. 4A.

FIG. 5 is a flowchart illustrating how protocol status information isobtained from intercepting a protocol request from a protocol client anda protocol reply from a protocol server and how this protocol statusinformation is then used to verify other protocol messages, according toone embodiment. At 500, a protocol message is intercepted. The protocolmessage contains a client request for network services that is beingsent from a protocol client to a protocol client via a network. Inresponse to the intercepted client request protocol message, protocolstatus information is updated at 502 to include information in theprotocol message and/or to identify the interface that received theclient request. The protocol status information can also be updated toindicate that the protocol client has an outstanding protocol request.

At 504, a protocol message that contains a protocol server reply isintercepted as the protocol message is being sent from the protocolserver to the protocol client. In response, the protocol statusinformation is updated at 506 to include information in the server replyand/or to match the server reply to the outstanding client requestintercepted at 500.

Dependent upon the protocol status information updated at 502 and 506, asecurity action is performed at 508 in response to intercepting asubsequent protocol message. A security action can involve dropping thesubsequent protocol message or logging an error. For example, theprotocol status information can be used to verify the identity of thesender of the subsequent protocol message, or to determine that a presetrate limit of protocol messages has not been exceeded. Note that theprotocol status information can also be updated in response to thesubsequent protocol message. In some embodiments, the protocol statusinformation updated at 502 and 506 is also used to determine whether toperform a security action in response to intercepting a subsequentnon-protocol message, as indicated at 510.

Message Filtering Based on Protocol Snooping

As noted above, the information in the binding table can also be used tofilter non-protocol messages. For example, a binding table that storesIP source address and MAC source address for a particular interface canbe used to filter non-protocol messages received via that interface. Byfiltering non-protocol messages, additional security can be provided.For example, if binding table entries store IP source addresses,additional security for LP messages can be provided, so that attackersare inhibited from stealing and using (also referred to as “spoofing”)the IP address of an innocent user.

It is noted that, as used herein, “protocol messages” are messages thatare conveyed according to a protocol (e.g., DHCP) that is used to assignnetwork addresses to protocol clients. “Non-protocol messages” aremessages that are conveyed via a protocol (e.g., IP, IPX (InternetworkPacket Exchange), and the like) other than the protocol that is used toassign network addresses to protocol clients.

FIG. 6 shows an example of a switch that uses information collected by asnooping agent to filter non-protocol messages. As shown, switch 20includes several interfaces 22A-22 n, as well as an access controlmodule 602, a processing module 604, and a binding table 70. Inputmessages are received via interfaces 22A-22 n, and control processingfor those messages is provided by (at least) access control module 602,processing module 604, and binding table 70. Each interface 22A-22 n canbe a logical interface (e.g., implemented using EtherChannel) orphysical interface. As shown by the arrows, information associated withand/or included in each message received via interfaces 22A-22 n can beprovided to access control module 602 and/or processing module 604.Binding table 70 includes information obtained by snooping protocol(e.g., DHCP) messages, as described above.

Access control module 602 includes an access control list 610. Accesscontrol list 610 stores information encoding access control rules.Access control module 602 uses information associated with each incomingmessage to identify which access control rules, if any, in accesscontrol list 610 should be applied to that incoming message. Forexample, based on information (e.g., source address, destinationaddress, and VLAN) in the message's IP header, access control module 602can select that a particular set of access control rules should beapplied to the message. Based on whether a message complies with aselected access control rule, access control module 602 can determinewhether the message should be dropped, sent to processing module 604 foradditional processing, allowed to be routed and/or forwarded normally,and the like. Access control module 602 can output one or more signals(not shown) indicating whether a message should be dropped, receiveadditional processing by processing module 604, and/or be routed and/orforwarded normally.

In some embodiments, access control module 602 is implemented usingternary content addressable memory (TCAM) to compare information (orselected subsets of that information, as determined by a mask includedin each TCAM entry) included in and/or associated with a given messageto information that is used to select one or more access control rules.A lookup in the TCAM can select one or more entries in the accesscontrol list 610, and the selected entries store information encodingthe access control rule to apply to the message.

Processing module 604 can be implemented by one or more processors(e.g., microprocessors, programmable logic devices, application-specificintegrated circuits, or the like) and/or memory devices. Processingmodule 604 can execute software and/or include application specifichardware for performing various tasks, such as implementing snoopingagent 50 (as shown in FIG. 2A). Processing module 604 can processinformation associated with and/or included in messages that areselected by access control module 602. For example, access controlmodule 602 can implement protocol message detector 24 of FIG. 2A, andprocessing module 604 can implement snooping agent 50 when handlingprotocol messages 202 detected by protocol message detector 24.

Processing module 604 is coupled to access information in binding table70. In this example, each time a new entry is allocated in binding table70, processing module 604 allocates or modifies a corresponding entry inaccess control list 610. Similarly, if entries are deallocated inbinding table 70, processing module 604 can remove or modify thecorresponding entries in access control list 610. For example, ifsnooping agent 50 detects a DHCP ACK message specifying a particular IPaddress and allocates a binding table entry for that IP address inbinding table 70, processing module 604 can update a corresponding entryin access control list 610. Similarly, if snooping agent 50 detects aDHCP RELEASE message for an IP address and, in response, removes orupdates the binding table entry for the IP address, processing module604 can remove the corresponding access control list entry in accesscontrol list 610. If a binding table entry is modified, processingmodule 604 can update the corresponding entry in access control list610.

Updating an entry in access control list 610 involves calculating anaccess control rule based on the information in a binding table entryand then storing the access control rule in access control list 610.Each binding table entry associates an network source address with oneof interfaces 22A-22 n. For example, if the protocol being snooped isused to assign IP address A to a client coupled to switch 20 viainterface 22A, a binding table entry can be allocated to store theinterface ID of interface 22A and IP address A. It is noted that bindingtable entries can also store additional information, such as informationidentifying the client device's MAC address and VLAN.

In one embodiment, at least some access control rules are calculated ona per-interface and per-VLAN basis. Initially, the access control rulefor a particular VLAN on a particular interface can indicate that allnon-protocol messages (where the protocol is the protocol monitored byprotocol message detector 24) should be dropped. Thus, an entry, whichcorresponds to that interface and VLAN, in access control list 610indicates that all non-protocol messages received via that interface inthat VLAN should be dropped. Another entry can store informationencoding a rule that all protocol messages should be passed to snoopingagent 50 for handling. Thus, when switch 20 begins operation, allnetwork messages that are not protocol messages will be dropped.

In response to snooping the protocol messages received via thatinterface, a binding table entry is allocated for that interface andVLAN. In response to the binding table entry, a new access control rulefor the interface and VLAN is calculated. This new access control rulecombines the pre-existing rule, which indicates that all non-protocolmessages should be dropped, with a new rule. The new rule indicates thatthat all messages received via that interface and VLAN that are notprotocol messages or messages having the source address specified in thebinding table entry should be dropped. Thus, in response to the bindingtable entry, the access control rule is updated to allow messages havingthat source address to be received via the interface (i.e., the updatedaccess control rule will not cause such messages to be dropped). Similarupdates can be applied to the access control rule each timecorresponding binding table entries are modified or removed.

Switch 20 can allow a user to select whether network traffic filtering,using access control rules based on entries in binding table 70, isenabled. If the network traffic filtering is enabled, access controlrules such as those described above will be calculated based on bindingtable 70. If the network traffic filtering is not enabled, these accesscontrol rules will not be calculated. In one embodiment, when switch 20is powered on, network traffic filtering is disabled. If a user enablesnetwork traffic filtering, a default access control rule (e.g.,specifying that all non-protocol messages should be dropped) iscalculated for each interface that is coupled to a protocol clientand/or each VLAN on which network traffic filtering is enabled. Theseaccess control rules are then updated based on binding table 70, asdescribed above. If a user disables network traffic filtering duringoperation of switch 20, any access control rules calculated from entriesin binding table 70 can be deleted or recalculated. In some embodiments,network traffic filtering can be enabled and disabled on a per-interfaceand/or per-VLAN basis. Processing module 604 maintains state information(e.g., in a memory device, not shown) that indicates whether networktraffic filtering has been enabled and updates this state information inresponse to user input to switch 20.

As described above, binding table entries store information associatingan network address, assigned by a protocol server, with one ofinterfaces 22A-22 n of switch 20. When such a binding table entry isallocated by snooping agent 50 (shown in FIG. 2A), processing modulegenerates information encoding a rule that allows network traffic fromthe source address, as identified in the binding table entry, to bereceived from the interface identified in the binding table entry. Thisinformation is stored in an entry in access control list. Subsequently,messages that are received via the identified entry and that have theidentified source address will not be dropped.

It is noted that in addition to allocating binding table entries inresponse to snooped protocol traffic, binding table entries can also beallocated in response to other actions that that associate a networkaddress with a particular interface. For example, a snooping agent 50can also be configured to allocate binding table entries in response tostatically configured network addresses (e.g., IP addresses manuallyassociated with a particular interface by a system administrator).Corresponding access control rules can also be updated based on bindingtable entries that are allocated in response to statically configurednetwork addresses. Additionally, binding table entries can be allocatedin response to other types of snooped protocol traffic, in addition toand/or instead of DHCP, or even in response to information detectedwithin certain non-protocol messages.

Access control module 602 filters messages (e.g., by causing certainmessages to be dropped) that are received via a particular interface andhave a source address other than the address identified in the accesscontrol rule for that interface. This prevents a malicious client devicefrom attacking the network by hijacking a neighboring client device'snetwork address. In some embodiments, network traffic filteringperformed using access control module 610 provides per-interface networktraffic filtering at “wire speed,” such that network traffic filteringhas a negligible effect on the performance of switch 20.

As an example, the following rules can be applied to a particularinterface if snooping agent 50 has detected that IP addresses IP1 andIP2 are assigned to client devices coupled to that interface: (1)capture all DHCP messages; (2) permit IP traffic from source IP1; (3)permit IP traffic from source IP2; and (4) deny all IP traffic. Rule (1)can be specified in such a way that Rule (1) applies to all interfacesfor which protocol snooping has been enabled (i.e., a single accesscontrol rule, stored in a single entry of access control list 610, canbe used to enable the capturing of protocol messages received via allinterfaces). Rule (4) can be encoded in access control list in responseto IP traffic filtering being enabled on the interface. A user canenable IP traffic filtering on a particular interface by specifying theparticular interface or by specifying the VLAN that includes thatinterface. If a malevolent client device coupled to that interfaceattempts to use another IP address (e.g., IP3), messages sent to switch20 by that malevolent client device will be dropped.

In addition to dropping messages that do not match information inbinding table 70, access control module 604 can also perform othersecurity actions. For example, access control module 604 can storeinformation corresponding to each dropped message in a log. Anadministrator can access such a log in order to obtain information foruse in analyzing a network attack. As another example, access controlmodule 604 can cause rate-limited system messages to be displayed on aconsole and/or sent to an administrator (e.g., using simple networkmanagement protocol (SNMP)) each time a message (or number or rate ofmessages) is dropped.

In some embodiments, more than one protocol client can share the sameaccess interface. Accordingly, there can be more than one networkaddress associated with each interface in binding table 70. Networktraffic filtering for all source addresses associated with a giveninterface can be performed via a single rule encoded in access controllist.

In some embodiments, processing module 604 calculates access controlrules that filter network traffic based on both a Layer 3 (L3) (e.g.,IP, IPX, and the like) source address and a Layer 2 (L2) (e.g., MAC)source address. For example, if an entry in binding table 70 identifiesinterface 22B, source IP address IP1, and source MAC address MAC-C,processing module can encode a corresponding access control rule inaccess control list 610. The corresponding access control rule indicatesthat messages received via interface 22B will be dropped, unless thosemessages are either protocol messages or messages having source IPaddress IP1 and source MAC address MAC-C.

In an alternative embodiment, the access control rules encoded in accesscontrol list 610 only provide source address filtering based on sourceL3 address (as opposed to providing source address filtering based onboth L3 source address and L2 source address). In some of thesealternative embodiments, filtering based on L2 source addresses is notprovided. In other embodiments, a separate module (other than accesscontrol module 602) is included in switch 20 to perform filtering basedon L2 source addresses. For example, a separate utility (e.g., the portsecurity feature of CISCO IOS™ Software, available from Cisco Systems,Inc. of San Jose, Calif.) is used to filter messages received via theinterface based on L2 source addresses. This separate utility can usethe information in binding table 70 to perform filtering based on L2source addresses.

It is noted that although the description of FIG. 6 refers to using theinformation in binding table 70 to snoop IP messages, other embodimentscan use similar binding tables to filter other types of non-protocolmessages. It is also noted that different protocols (in addition toand/or instead of DHCP) can be snooped for information to include in abinding table.

In some embodiments, a switch implements the functionality of severaldifferent snooping agents, binding tables, and access control modules inorder to perform network traffic filtering for several different networkprotocols. For example, as shown in FIG. 6A, a switch 20 can implementthe functionality of a pair of snooping agents 50(1) and 50(2), a pairof binding tables 70(1) and 70(2), and a pair of access control modules602(1) and 602(2). In this example, snooping agent 50(1) snoops DHCPv4protocol messages, which are used to assign IPv4 (Internet Protocolversion 4) addresses to DHCPv4 clients. Based on snooped DHCPv4messages, snooping agent 50(1) allocated entries in binding table 70(1).The information in binding table 70(1) can then be used by accesscontrol module 602(1) to filter IPv4 network traffic.

Similarly, snooping agent 50(2) snoops DHCPv6 protocol messages, whichare used to assign IPv6 (Internet Protocol version 6) addresses toDHCPv6 clients. Based on snooped DHCPv6 messages, snooping agent 50(2)allocated entries in binding table 70(2). The information in bindingtable 70(2) can then be used by access control module 602(2) to filterIPv6 network traffic.

Thus, as shown in FIG. 6A, a network device such as switch 20 canperform traffic filtering for several different network protocols bysnooping each of several different protocols that are used to assignnetwork addresses to clients. It is noted that the different snoopingagents, binding tables, and access control modules can be physicallyseparate in some embodiments, and integrated in other embodiments. Also,the functionality of more than two sets of snooping agents, bindingtables, and access control modules can be implemented.

FIG. 7 illustrates how the functionality used to implement IP trafficfiltering can be implemented in some embodiments. In the example of FIG.7, switch 20 includes a forwarding module 702 that performs forwardinglookups (e.g., based on Ethernet headers) for messages received byswitch 20. Switch 20 also includes access control module 602, an addressfiltering module 704, and snooping agent 50. In one embodiment,forwarding module 702 and access control module 602 are implemented inhardware, while address filtering module 704 and snooping agent 50 areimplemented in software. The modules can be implemented differently(e.g., all in software) in other embodiments.

A message received by switch 20 is initially processed based oninformation in forwarding module 702 (e.g., by providing informationassociated with and/or included in the message to forwarding module702). Forwarding module 702 stores addresses (e.g., MAC addresses) thatare included in messages handled by switch 20. In one embodiment,forwarding module 702 stores information associating MAC addresses withinterface IDs. For example, each time a message is received by switch20, the source address of the message and the interface ID of theinterface via which the message entered switch 20 can be stored in alookup table included in forwarding module 702. If a subsequentlyreceived message specifies the stored address as a destination address,the subsequently received message can be forwarded to the interfaceidentified by the stored interface ID. Forwarding module 702 can be usedto perform L2 (e.g., Ethernet) forwarding within switch 20.

If the source address of an incoming message is not found by forwardingmodule 702 (i.e., if there is no matching entry for that message, basedon the message's source address, in the lookup table), informationincluded in and/or associated with the incoming message is passed toaddress filtering module 704 for additional processing. If the sourceaddress of the incoming message hits in the lookup table (i.e., if amatching entry for the source address of that message is found),information included in and/or associated with the message can beprovided to access control module 602. Access control module 602 canprovide information to forwarding module 702 indicating whether a givenmessage should be dropped or provided to snooping agent 50 foradditional handling.

As described above, access control module 602 stores informationencoding various access control rules, which can be calculated based oninformation in a binding table. One of the access control rules canspecify that all protocol messages (e.g., all DHCP messages) should behandled in software (e.g., to perform protocol message snooping). Thus,as shown in FIG. 7, protocol messages are detected by access controlmodule 602. When a protocol message is detected, information associatedwith and/or included in the protocol messages is then provided tosnooping agent 50 (e.g., in response to an access control rule).Additional access control rules in access control module 602 can specifywhich source addresses are allowed to be received via various interfacesin switch 20. Access control module 602 can operate to drop messageshaving source addresses that are not explicitly allowed by an accesscontrol rule associated with the interface that received those messages.

Snooping agent 50 can drop protocol messages (e.g., based on ratelimiting constraints or based on information in a binding table createdby snooping agent 50), create or modify binding table entries based onprotocol messages, and/or allow protocol messages to be output. Snoopingagent 50 can operate as described above with respect to FIG. 2A.

Address filtering module 704 performs processing for messages thatcannot be processed in hardware (e.g., due to the source address of amessage missing in a lookup table). In the illustrated example, addressfiltering module 704 performs IP source address and layer 2 (L2) sourceaddress filtering. The L2 source address filtering can, for example,involving filtering messages based on MAC source addresses included inEthernet headers. In this example, L2 source address filtering isperformed by an L2 address filtering module 706, and L3 source addressfiltering is performed by an L3 address filtering module 708. It isnoted that while L2 address filtering module 706 and L3 addressfiltering module 708 are implemented separately and operateindependently of each other in this example, the functionality of thesemodules can be integrated and/or inter-dependent in other embodiments.

L3 address filtering module 708 detects protocol messages (e.g., DHCPmessages) and passes information associated with and/or included indetected protocol messages to snooping agent 50. L3 address filteringmodule 708 then filters non-protocol messages based on information inbinding table 70. For example, if a binding table entry associates asingle IP address IP1 with interface ID I12, L3 address filtering module708 will not drop a message having a source address of IP1 and receivedvia 112. However, L3 address filtering module 708 will cause a messagehaving a source address of IP2 that was received via 112 to be dropped.Thus, L3 address filtering module 708 drops any non-protocol messagesthat are not consistent with information in binding table 70.

L2 address filtering module 706 can also filter messages based oninformation in binding table 70. Alternatively, L2 address filteringmodule 706 can maintain its own binding table, based on informationobtained by snooping agent 50.

Some embodiments can perform at least some of the traffic filtering insoftware. For example, address filtering module 704 can be implementedin software and used to perform processing for messages having sourceaddresses that miss in the lookup table of forwarding module 702. Insuch embodiments, performing processing for messages in software mayconsume more time and/or switch resources than would be needed toperforming corresponding processing for those messages in hardware. Thispresents a security weakness that a malicious user can exploit bysending a large number of messages, constructed in a way that willrequire software handling, to the switch. If the switch attempts tohandle all of the malicious user's messages, the performance of theswitch may be negatively affected. In order to prevent such attacks fromhaving a negative impact, address filtering module 704 can implementrate limiting. Rate limiting operates such that if a certain number of“bad” messages (i.e., messages that are filtered from the messagestream) are detected within a given time period, the module will begindropping messages provided to the module, without first evaluating thosemessages. Rate limiting can be performed on a per-interface basis (e.g.,if rate limiting is triggered, all messages received via a particularinterface will be dropped), a per-source basis (e.g., if rate limitingis triggered, all messages received from a particular source will bedropped), and/or any other appropriate basis.

FIG. 8 illustrates one embodiment of a method of performing trafficfiltering based on information obtained via protocol snooping. At 810,an access control rule is installed (e.g., by allocating an entry in anaccess control list to store information encoding the access controlrule). The access control rule specifies that protocol messages (e.g.,all DHCP messages) received via one or more interfaces should becaptured. When such messages are captured, information associated withand/or included in those messages is provided to a snooping agent (e.g.,such as snooping agent 50 described above) that verifies the protocolmessages and/or stores information in the protocol messages in a bindingtable. The binding table identifies network addresses that have beenassigned to clients that are coupled to particular interfaces. In someembodiments, the binding table identifies L2 and L3 addresses of theclients and other information, as described above.

The access control rule installed at 810 can specify one interface, asubset of all of the interfaces in a given network device, or all of theinterfaces in the network device. For example, the access control rulecan specify all of the interfaces in the network device for whichprotocol snooping and/or traffic filtering has been enabled. Thus, asingle access control rule can be used to capture protocol messagesreceived via more than one interface.

At 820, another access control rule is installed. This access controlrule indicates that non-protocol messages received via one or moreinterfaces (e.g., interfaces for which traffic filtering has beenenabled) should be dropped. Accordingly, protocol messages will becaptured (due to the rule installed at 810) and non-protocol messageswill be dropped. It is noted that installing the access control rule at820 can involve installing a respective access control rule for each ofthe one or more interfaces for which traffic filtering is enabled.

Binding table entries are updated (e.g., allocated, modified and/ordeleted) in response to captured protocol messages. Each time a bindingtable entry is updated, as detected at 830, an access control rulecorresponding to the binding table entry is updated (e.g., byallocating, modifying, and/or deleting an entry in an access controllist), as shown at 840. For example, if a new binding table entry isallocated to store information indicating that IP address IP1 has beenassigned to a client coupled to interface I1, a corresponding entry inan access control list can be allocated to store information encoding arule stating that messages having source address IP1 that are receivedvia interface I1 should be allowed. Alternatively, an existing entry inthe access control list that corresponds to interface I1 can bemodified. For example, an entry that stores the access control ruleinstalled at 820 for interface I1 can be modified, such that themodified rule indicates that all messages, other than the messageshaving source address IP1, that are received via interface I1 should bedropped. It is noted that the corresponding access control rule can alsospecify other information (e.g., VLAN, MAC address, and the like)included in the binding table entry. For example, the correspondingaccess control rule could state that all messages received via aparticular interface, except messages having the IP source address andMAC source address indicated in the binding table entry associated withthat interface, should be dropped.

If a message is received via one of the interfaces, as detected at 850,the access control rules associated with that interface (e.g., asdetected using a content addressable memory to compare information inthe message header or headers to information used to select appropriateaccess control rules) are applied to the message, as shown at 860. Thus,if the access control rules indicate that the message should only beallowed if the source IP address of the message is IP1 and the sourceMAC address of the message is MAC 3, the message is dropped or notdropped based on the source IP and MAC addresses of the message.

FIG. 9 illustrates how a private VLAN can be implemented in a network.As shown, two protocol clients (e.g., two DHCP clients) 12(1) and 12(2)are coupled to a protocol server 16 by (at least) one network device,switch 20. Switch 20 performs IP message filtering as described above(e.g., using access control list 610 of FIG. 6). In this example, eachprotocol client 12(1) and 12(2) is coupled to switch 20 by a differentisolated port within secondary VLAN 900. In particular, protocol client12(1) is coupled to switch 20 by isolated port 902(1) of secondary VLAN900, while protocol client 12(2) is coupled to switch 20 by isolatedport 902(2) of secondary VLAN 900. Secondary VLAN 900 is implemented asa private VLAN within a primary VLAN 910, which conveys traffic betweenprotocol clients 12(1) and 12(2) and protocol server 16. Secondary VLAN900 is private in that clients 12(1) and 12(2) cannot communicatedirectly with each other via secondary VLAN 900. Messages generated byprotocol clients 12(1) and 12(2) are conveyed in secondary VLAN 900.Messages generated by protocol server 16 are conveyed in primary VLAN910.

Private VLANs are often used to provide isolation between differentinterfaces of a network device such as switch 20. Private VLANs aredefined by configuring interfaces as promiscuous, isolated, and/orcommunity interfaces and mapping each interface to a primary orsecondary VLAN (isolated interfaces are illustrated in FIG. 9). Isolatedand community interfaces are part of secondary VLANs, while promiscuousinterfaces are part of primary VLANs. In this example, the interfacecoupled to protocol server 16 is a promiscuous interface. The interfaces902(1) and 902(2) coupled to the protocol clients 12(1) and 12(2) areisolated interfaces.

Messages received via a promiscuous interface in a primary VLAN can beconveyed via any other isolated, community, or promiscuous interfaces inthe same primary VLAN. Thus, in FIG. 9, messages received by switch 20from protocol server 16 can be conveyed to any of the isolatedinterfaces in secondary VLAN 900 (depending on other criteria associatedwith such messages, which may be used to select a particular interfacewithin those secondary VLANs).

In contrast, messages that are received by switch 20 from an isolated orcommunity interface in a secondary VLAN can only be sent via promiscuousinterfaces in the same primary VLAN and community interfaces in the samesecondary VLAN. Isolated interfaces that are part of the same secondaryVLAN cannot exchange messages (i.e., a messages received via an isolatedinterface cannot be sent via another isolated interface, even if theother isolated interface is in the same secondary VLAN as the receivinginterface). Community interfaces can only exchange messages withpromiscuous interfaces in the same primary VLAN and other isolated andcommunity interfaces in the same secondary VLAN.

As described above, a binding table entry (e.g., in binding table 70 ofFIG. 6) is typically allocated in response to a message (e.g., a DHCPACK) being conveyed from a protocol server to a protocol client. Asshown in FIG. 9, such messages are conveyed in the primary VLAN 910.

The information in the binding table can then be used to filter messagesbeing conveyed from the protocol clients 12(1) and/or 12(2), which areconveyed in secondary VLAN 900. Thus, by snooping messages conveyed inone VLAN (e.g., primary VLAN 910), information is obtained for use infiltering messages being conveyed in another VLAN (e.g., secondary VLAN900). Looking back at FIG. 6, processing module 604 can calculate accesscontrol rules in response to binding table entries that specify theprimary VLAN and an interface ID. Processing module 604 calculates andencodes the access control rules such that the rules specify and/or areapplied to a secondary VLAN instead of the primary VLAN. For example, abinding table entry associating IP address IP1 with the interfacecoupled to protocol client 12(2) can be allocated in response toprotocol server 16 assigning IP address IP1 to protocol client 12(2) bysending a DHCP ACK message in primary VLAN 910. Processing module 604can calculate an access control rule indicating that messages receivedvia the interface coupled to protocol client 12(2) in secondary VLAN 900should be dropped unless the source IP address of those messages is IP1.

FIG. 10 is a block diagram of switch 20 illustrating how a snoopingagent 50 and access control rule update module 1050 (which updatesaccess control rules in response to information obtained by snoopingagent 50, using the techniques described above) and can be implementedin software executing on a network device such as switch 20. It is notedthat other components (e.g., access control module 602, as shown in FIG.6) can also be implemented in software executing on switch 20 in someembodiments. As illustrated, switch 20 includes one or more processors1010 (e.g., implemented using one or more microprocessors, PLDs(Programmable Logic Devices), ASICs (Application Specific IntegratedCircuits), or the like) configured to execute program instructionsstored in memory 1020. Memory 1020 can include various types of RAM,ROM, Flash memory, MEMS memory, and the like. In the illustratedembodiment, access control module 602 is implemented in hardware (e.g.,using a CAM and/or ACL). Processor 1010, access control module 602, andmemory 1020 are coupled to send and receive data and control signals bya bus or other interconnect.

Memory 1020 stores program instructions executable by processor 1010 toimplement an operating system 1022 configured to control basicfunctionality of switch 20. Operating system 1022 can include snoopingagent 50 and access control rule update module 1050 (alternatively,snooping agent 50 and access control rule update module 1050 can beimplemented as programs that interact with, but is not part of,operating system 1022). Memory 1020 may also be used to store protocolstatus information 60 used and maintained by snooping agent 50.

The program instructions and data implementing snooping agent 50 andaccess control rule update module 1050 can be stored upon variouscomputer readable media such as memory 1020. In some embodiments,snooping agent 50 and access control rule update module 1050 software isstored on a computer readable medium such as a CD (Compact Disc), DVD(Digital Versatile Disc), hard disk, optical disk, tape device, floppydisk, and the like). In order be executed by processor 1010, theinstructions and data implementing snooping agent 50 and access controlrule update module 1050 are loaded into memory 1020 from the othercomputer readable medium. The instructions and/or data implementing canalso be transferred to switch 20 via a network such as the Internet orupon a carrier medium. In some embodiments, a computer readable mediumis a carrier medium such as a network and/or a wireless link upon whichsignals such as electrical, electromagnetic, or digital signals, onwhich the data and instructions implementing snooping agent 50 andaccess control rule update module 1050 are encoded, are conveyed.

Although the present invention has been described with respect tospecific embodiments thereof, various changes and modifications may besuggested to one skilled in the art. It is intended such changes andmodifications fall within the scope of the appended claims.

What is claimed is:
 1. A method comprising: applying an access controlrule to a first message, wherein the first message comprises messageinformation, the message information is used to select the accesscontrol rule, the access control rule is calculated using protocolstatus information, the protocol status information is obtained inresponse to one or more protocol messages sent between at least oneclient and a protocol server, the protocol status information comprisesprotocol information generated by the protocol server, the one or moreprotocol messages are conveyed according to a protocol used to assignnetwork addresses to clients, and the access control rule is stored inan access control list; determining whether to perform one or moresecurity actions, wherein the determining is performed in response tothe applying, and the access control rule indicates whether to performthe one or more security actions; and performing a first security actionfor the first message, wherein the access control rule indicatesperformance of the first security action; and selecting to unicast thefirst message instead of forwarding the first message normally, whereinthe first message would normally be broadcast, multicast, or flooded tomultiple recipients, and the selecting to unicast the first message isperformed in response to a determination that the first messagecomprises a first protocol message from the protocol server, updating abinding table entry in response to detecting the one or more protocolmessages, wherein the binding table entry comprises the protocol statusinformation corresponding to the client, and the protocol statusinformation identifies an Internet Protocol (IP) address of the client,a Media Access Control (MAC) address of the client, and an interfacecoupled to the client, and allocating an entry in the access controllist to store access control information, wherein the access controlinformation encodes a second access control rule, the second accesscontrol rule requires that a Dynamic Host Configuration Protocol (DHCP)message received via the interface identified in the protocol statusinformation be processed by a snooping agent, and the snooping agent isconfigured to update information in the binding table in response toprocessing the DHCP message.
 2. The method of claim 1, wherein theaccess control rule requires that the first message be discarded unlessthe message information matches the protocol status information, and thefirst message is discarded, in response to the access control rule. 3.The method of claim 2, further comprising: providing a messagecorresponding to the first message to an administrator, in response todetermining that the first message should be discarded.
 4. The method ofclaim 1, wherein the first message is received via the interfaceidentified in the protocol status information, and the access controlrule requires that the first message be dropped unless the messageinformation comprises a IP source address that matches the IP address ofthe client identified in the protocol status information.
 5. The methodof claim 1, wherein the first message is received via the interfaceidentified in the protocol status information, and the access controlrule requires that the first message be dropped unless the messageinformation comprises a MAC source address that matches the MAC addressof the client identified in the protocol status information.
 6. Themethod of claim 1, further comprising: updating an entry in the accesscontrol list to store information encoding the access control rule,wherein the updating the entry in the access control list is performedin response to detecting that the binding table entry has been updated.7. The method of claim 1, wherein a second access control rule is storedin the access control list, the second access control rule requires thata Dynamic Host Configuration Protocol (DHCP) message received via anyone of a plurality of interfaces be processed by a snooping agent, andaccess control information encoding the second access control rule isstored in a single entry in the access control list.
 8. The method ofclaim 1, wherein the binding table entry is updated in response todetecting a first protocol message, and the protocol server comprises aDHCP server.
 9. A network device comprising: a physical interfaceconfigured to receive a first message; a filtering module, wherein thefiltering module is configured to apply an access control rule to thefirst message, the first message comprises message information, thefiltering module is configured to select the access control rule usingthe message information, and the filtering module is configured todetermine whether to perform one or more security actions, in responseto applying the access control rule to the first message, and perform afirst security action for the first message, wherein the access controlrule indicates performance of the first security action, and thefiltering module is configured to select to unicast the first messageinstead of forwarding the first message normally, wherein the firstmessage would normally be broadcast, multicast, or flooded to multiplerecipients, and the selecting to unicast the first message is performedin response to a determination that the first message comprises a firstprotocol message from the protocol server; and a binding table, whereinthe binding table is configured to store protocol status information,the protocol status information is obtained in response to one or moreprotocol messages sent between at least one client and a protocolserver, the protocol status information comprises protocol informationgenerated by the protocol server, and the one or more protocol messagesare conveyed according to a protocol used to assign network addresses toclients; and an access control list, wherein the access control rule isstored in the access control list, the access control rule is calculatedusing the protocol status information, and the access control ruleindicates whether to perform the one or more security actions; asnooping agent, wherein the snooping agent is configured to update abinding table entry in response to detecting the one or more protocolmessages, the binding table entry comprises the protocol statusinformation corresponding to the client, and the protocol statusinformation identifies an Internet Protocol (IP) address of the client,a Media Access Control (MAC) address of the client, and an interfacecoupled to the client, an access control rule update module, wherein theaccess control rule update module is configured to allocate an entry inthe access control list to store access control information, the accesscontrol information encodes a second access control rule, the secondaccess control rule requires that a Dynamic Host Configuration Protocol(DHCP) message received via the interface identified in the protocolstatus information be processed by the snooping agent, and the snoopingagent is configured to update information in the binding table inresponse to processing the DHCP message; and a memory having storedthereon the filtering module, snooping agent, the access control moduleand the binding table.
 10. The network device of claim 9, wherein theaccess control rule requires that the first message be discarded unlessthe message information matches the protocol status information, and thefirst message is discarded, in response to the access control rule. 11.The network device of claim 10, wherein the filtering module is furtherconfigured to provide a message corresponding to the first message to anadministrator, in response to determining that the first message shouldbe discarded.
 12. The network device of claim 9, wherein the firstmessage is received via the interface identified in the protocol statusinformation, and the access control rule requires that the first messagebe dropped unless the message information comprises a IP source addressthat matches the IP address of the client identified in the protocolstatus information.
 13. The network device of claim 9, wherein the firstmessage is received via the interface identified in the protocol statusinformation, and the access control rule requires that the first messageto be dropped unless the message information comprises a MAC sourceaddress that matches the MAC address of the client identified in theprotocol status information.
 14. The network device of claim 9, furthercomprising: an access control rule update module, wherein the accesscontrol rule update module is configured to update an entry in theaccess control list to store information encoding the access controlrule, in response to detecting that the binding table entry has beenupdated.
 15. The network device of claim 9, wherein a second accesscontrol rule is stored in the access control list, the second accesscontrol rule requires that a Dynamic Host Configuration Protocol (DHCP)message received via any one of a plurality of interfaces be processedby the snooping agent, and access control information encoding thesecond access control rule is stored in a single entry in the accesscontrol list.
 16. The network device of claim 9, wherein the snoopingagent is configured to update the binding table entry in response todetecting a first protocol message, and the protocol server comprises aDHCP server.
 17. A system comprising: means for applying an accesscontrol rule to a first message, wherein the first message comprisesmessage information, and the means for applying comprises means forselecting the access control rule using the message information; meansfor obtaining protocol status information in response to means fordetecting one or more protocol messages sent between at least one clientand a protocol server, wherein the protocol status information comprisesprotocol information generated by the protocol server, and the one ormore second messages are conveyed according to a protocol used to assignnetwork addresses to clients; means for storing access control rules,wherein the means for storing access control rules stores the accesscontrol rule; means for calculating the access control rule using theprotocol status information, wherein the access control rule indicateswhether to perform one or more security actions; means for storing theprotocol status information; means for determining whether to performthe one or more security actions, in response to the means for applyingthe access control rule to the first message; means for performing afirst security action for the first message, wherein the access controlrule indicates performance of the first security action; and means forselecting to unicast the first message instead of forwarding the firstmessage normally, wherein the first message would normally be broadcast,multicast, or flooded to multiple recipients, and the selecting tounicast the first message is performed in response to a determinationthat the first message comprises a first protocol message from theprotocol; means for updating a binding table entry in response to themeans for detecting the one or more protocol messages, wherein thebinding table entry comprises the protocol status informationcorresponding to the client, and the protocol status informationidentifies an Internet Protocol (IP) address of the client, a MediaAccess Control (MAC) address of the client, and an interface coupled tothe client; means for allocating an entry in the means for storingaccess control rules to store access control information, wherein theaccess control information encodes a second access control rule, thesecond access control rule requires that a Dynamic Host ConfigurationProtocol (DHCP) message received via the interface identified in theprotocol status information be processed by the means for updating thebinding table entry.
 18. The system of claim 17, wherein the accesscontrol rule requires that the first message be discarded unless themessage information matches the protocol information, and the firstmessage is discarded, in response to the access control rule.
 19. Thesystem of claim 18, further comprising: means for providing a messagecorresponding to the first message to an administrator, in response todetermining that the first message should be discarded.
 20. The systemof claim 17, further comprising: means for receiving the first messagevia the interface identified in the protocol status information, whereinthe access control rule requires that the first message be droppedunless the message information comprises a IP source address thatmatches the IP address of the client identified in the protocol statusinformation.
 21. The system of claim 17, further comprising: means forreceiving the first message via the interface identified in the protocolstatus information, wherein the access control rule requires that thefirst message be dropped unless the message information comprises a MACsource address that matches the MAC address of the client identified inthe protocol status information.
 22. The system of claim 17, furthercomprising: means for updating an entry in the means for storing accesscontrol rules that stores information encoding the access control rule,in response to the means for updating the binding table entry.
 23. Thesystem of claim 17, wherein the means for storing access control rulesstores a second access control rule, the second access control rulefurther requires that a DHCP message received via any one of a pluralityof interfaces be processed by the means for updating the binding tableentry, and access control information encoding the second access controlrule is stored in a single entry in the means for storing access controlrules.
 24. The system of claim 17, further comprising: means forupdating a binding table entry in response to means for detecting afirst protocol message, wherein the protocol server comprises a DHCPserver.
 25. A computer readable non-transitory storage medium comprisingprogram instructions executable to: apply an access control rule to afirst message, wherein the first message comprises message information,the message information is used to select the access control rule, theaccess control rule is calculated using protocol status information, theprotocol status information is obtained in response to one or moreprotocol messages sent between at least one client and a protocolserver, the protocol status information comprises protocol informationgenerated by the protocol server, the one or more protocol messages areconveyed according to a protocol used to assign network addresses toclients, and the access control rule is stored in an access controllist; determine whether to perform one or more security actions, inresponse to applying the access control rule to the first message,wherein the access control rule indicates whether to perform the one ormore security actions; perform a first security action for the firstmessage, wherein the access control rule indicates performance of thefirst security action; and select to unicast the first message insteadof forwarding the first message normally, wherein the first messagewould normally be broadcast, multicast, or flooded to multiplerecipients, and the selecting to unicast the first message is performedin response to a determination that the first message comprises a firstprotocol message from the protocol server; update a binding table entryin response to detecting the one or more second messages, wherein thebinding table entry comprises the protocol status informationcorresponding to the client, and the protocol status informationidentifies an Internet Protocol (IP) address of the client, a MediaAccess Control (MAC) address of the client, and an interface coupled tothe client; allocate an entry in the access control list to store accesscontrol information, wherein the access control information encodes asecond access control rule, the second access control rule requires thata Dynamic Host Configuration Protocol (DHCP) message received via theinterface identified in the protocol status information be processed bya snooping agent, and the snooping agent is configured to updateinformation in the binding table in response to processing the DHCPmessage.
 26. The computer readable storage medium of claim 25, whereinthe access control rule requires that the first message be droppedunless the message information matches the protocol status information,and the first message is discarded, in response to the access controlrule.
 27. The computer readable storage medium of claim 26, wherein theprogram instructions are further executable to: provide a messagecorresponding to the first message to an administrator, in response todetermining that the first message should be discarded.
 28. The computerreadable storage medium of claim 25, wherein the first message isreceived via the interface identified in the protocol statusinformation, and the access control rule requires that the first messagebe dropped unless the message information comprises a IP source addressthat matches the IP address of the client identified in the protocolstatus information.
 29. The computer readable storage medium of claim25, wherein the first message is received via the interface identifiedin the protocol status information, and the access control rule requiresthe first message to be dropped unless the message information comprisesa MAC source address that matches the MAC address of the clientidentified in the protocol status information.
 30. The computer readablestorage medium of claim 25, wherein the program instructions are furtherexecutable to: update an entry in the access control list to storeinformation encoding the access control rule, wherein the updating theentry in the access control list is performed in response to detectingthat the binding table entry has been updated.
 31. The computer readablestorage medium of claim 25, wherein a second access control rule isstored in the access control list, the second access control rulerequires that a DHCP message received via any one of a plurality ofinterfaces be processed by a snooping agent, and access controlinformation encoding the second access control rule is stored in asingle entry in the access control list.
 32. The computer readablestorage medium of claim 25, wherein the binding table entry is updatedin response to detecting a first protocol message, and the protocolserver comprises a DHCP server.
 33. A method comprising: applying anaccess control rule to a first message, wherein the first messagecomprises message information the message information is used to selectthe access control rule, the access control rule is calculated usingprotocol status information, the protocol status information is obtainedfrom one or more protocol messages sent between at least one client anda protocol server, the protocol status information comprises protocolinformation generated by the protocol server, the one or more protocolmessages are conveyed according to a protocol used to assign a networkaddress to a client, and the access control rule is stored in an accesscontrol list; determining whether to perform one or more securityactions, wherein the determining is based on a result of the applying,and the access control rule indicates whether to perform the one or moresecurity actions; performing a first security action for the firstmessage, wherein the access control rule indicates performance of thefirst security action; and selecting to unicast the first messageinstead of forwarding the first message normally, wherein the firstmessage would normally be broadcast, multicast, or flooded to multiplerecipients, and the selecting to unicast the first message is performedin response to a determination that the first message comprises a firstprotocol message from the protocol server updating a binding table entryin response to detecting the one or more protocol messages, wherein thebinding table entry comprises the protocol status informationcorresponding to the client, and the protocol status informationidentifies an Internet Protocol (IP) address of the client, a MediaAccess Control (MAC) address of the client, and an interface coupled tothe client; allocating an entry in the access control list to storeaccess control information, wherein the access control informationencodes a second access control rule, the second access control rulerequires that a Dynamic Host Configuration Protocol (DHCP) messagereceived via any one of a plurality of interfaces be processed by asnooping agent, and the snooping agent is configured to updateinformation in the binding table in response to processing the DHCPmessage.
 34. The method of claim 33, wherein the access control rulerequires that the first message be discarded unless the messageinformation matches the protocol status information, and the firstmessage is discarded, in response to the access control rule.
 35. Themethod of claim 33, further comprising: updating an entry in the accesscontrol list to store information encoding the access control rule,wherein the first message is received via the interface identified inthe protocol status information, the access control rule requires thatthe first message be dropped unless the message information comprises aIP source address that matches the IP address of the client identifiedin the protocol status information, and the updating the entry in theaccess control list is performed in response to detecting that thebinding table entry has been updated.
 36. The method of claim 33,wherein the binding table entry is updated in response to detecting afirst protocol message, and the protocol server comprises a DHCP server.37. A network device comprising: a physical interface; a binding table,wherein the binding table is configured to store protocol statusinformation, the protocol status information is obtained from one ormore protocol messages sent between at least one client and a protocolserver, the protocol status information comprises protocol informationgenerated by the protocol server, and the one or more protocol messagesare conveyed according to a protocol used to assign a network address toa client; a filtering module coupled to the binding table, wherein thefiltering module is configured to apply an access control rule to afirst message, the first message comprises message information thefiltering module is configured to select the access control rule usingthe message information, and the filtering module is configured todetermine whether to perform one or more security actions, based on aresult generated by applying the access control rule to the firstmessage, perform a first security action for the first message, whereinthe access control rule indicates performance of the first securityaction, and the filtering module is configured to select to unicast thefirst message instead of forwarding the first message normally, whereinthe first message would normally be broadcast, multicast, or flooded tomultiple recipients, and the selecting to unicast the first message isperformed in response to a determination that the first messagecomprises a first protocol message from the protocol server; and anaccess control list, wherein the access control rule is stored in theaccess control list, the access control rule is calculated using theprotocol status information, and the access control rule indicateswhether to perform the one or more security actions; a snooping agent,wherein the snooping agent is configured to update a binding table entryin response to detecting the one or more protocol messages, the bindingtable entry comprises the protocol status information corresponding tothe client, and the protocol status information identifies an InternetProtocol (IP) address of the client, a Media Access Control (MAC)address of the client, and an interface coupled to the client; an accesscontrol rule update module, wherein the access control rule updatemodule is configured to allocate an entry in the access control list tostore access control information, the access control information encodesa second access control rule, the second access control rule requiresthat a Dynamic Host Configuration Protocol (DHCP) message received viaany one of a plurality of interfaces be processed by the snooping agent,and the snooping agent is configured to update information in thebinding table in response to processing the DHCP message; and a memoryhaving stored thereon the filtering module, snooping agent, the accesscontrol module and the binding table.
 38. The network device of claim37, wherein the access control rule requires that the first message bediscarded unless the message information matches the protocol statusinformation, and the first message is discarded, in response to theaccess control rule.
 39. The network device of claim 37, wherein thefirst message is received via the interface identified in the protocolstatus information, and the access control rule requires that the firstmessage be dropped unless the message information comprises a IP sourceaddress that matches the IP address of the client identified in theprotocol status information.
 40. The network device of claim 37, furthercomprising: an access control rule update module, wherein the accesscontrol rule update module is configured to update an entry in theaccess control list to store information encoding the access controlrule, in response to detecting that the binding table entry has beenupdated.
 41. The network device of claim 37, wherein the snooping agentis configured to update the binding table entry in response to detectinga first protocol message, and the protocol server comprises a DHCPserver.
 42. A system comprising: means for applying an access controlrule to a first message, wherein the first message comprises messageinformation, and the means for applying comprises means for selectingthe access control rule using the message information; means forobtaining protocol status information from one or more second messagessent between at least one client and a protocol server, wherein theprotocol status information comprises protocol information generated bythe protocol server, and the one or more second messages are conveyedaccording to a protocol used to assign network addresses to clients;means for storing access control rules, wherein the means for storingaccess control rules stores the access control rule; means forcalculating the access control rule using the protocol statusinformation, wherein the access control rule indicates whether toperform one or more security actions; means for storing the protocolstatus information; means for determining whether to perform the one ormore security actions, based on a result of the means for applying;means for performing a first security action for the first message,wherein the access control rule indicates performance of the firstsecurity action; and means for selecting to unicast the first messageinstead of forwarding the first message normally, wherein the firstmessage would normally be broadcast, multicast, or flooded to multiplerecipients, and the selecting to unicast the first message is performedin response to a determination that the first message comprises a firstprotocol message from the protocol server; means for updating a bindingtable entry in response to means for detecting the one or more protocolmessages, wherein the binding table entry comprises the protocol statusinformation corresponding to the client, and the protocol statusinformation identifies an Internet Protocol (IP) address of the client,a Media Access Control MAC) address of the client, and an interfacecoupled to the client; means for allocating an entry in the means forstoring access control rules to store access control information,wherein the access control information encodes a second access controlrule, the second access control rule requires that a Dynamic HostConfiguration Protocol (DHCP) message received via any one of aplurality of interfaces be processed by the means for updating thebinding table entry.
 43. The system of claim 42, wherein the accesscontrol rule requires that the first message be discarded unless themessage information matches the protocol information, and the firstmessage is discarded, in response to the access control rule.
 44. Thesystem of claim 42, further comprising: means for updating an entry inthe means for storing access control rules to store information encodingan access control rule, in response to the means for updating thebinding table entry, wherein the access control rule requires that thefirst message received via the interface be dropped unless the messageinformation comprises a IP source address that matches the IP address ofthe client identified in the protocol status information.
 45. The systemof claim 42, further comprising: means for updating a binding tableentry in response to means for detecting a first protocol message,wherein the protocol server comprises a DHCP server.
 46. A computerreadable non-transitory storage medium comprising program instructionsexecutable to: apply an access control rule to a first message, whereinthe first message comprises message information the message informationis used to select the access control rule, the access control rule iscalculated using protocol status information, the protocol statusinformation is obtained from one or more protocol messages sent betweenat least one client and a protocol server, the protocol statusinformation comprises protocol information generated by the protocolserver, the one or more protocol messages are conveyed according to aprotocol used to assign a network address to a client, and the accesscontrol rule is stored in an access control list; and determine whetherto perform one or more security actions, based on a result generated byapplying the access control rule to the first message, wherein theaccess control rule indicates whether to perform the one or moresecurity actions; perform a first security action for the first message,wherein the access control rule indicates performance of the firstsecurity action; and select to unicast the first message instead offorwarding the first message normally, wherein the first message wouldnormally be broadcast, multicast, or flooded to multiple recipients, andthe selecting to unicast the first message is performed in response to adetermination that the first message comprises a first protocol messagefrom the protocol server; update a binding table entry in response todetecting the one or more second messages, wherein the binding tableentry comprises the protocol status information corresponding to theclient, and the protocol status information identifies an InternetProtocol (IP) address of the client, a Media Access Control (MAC)address of the client, and an interface coupled to the client; allocatean entry in the access control list to store access control information,wherein the access control information encodes a second access controlrule, the second access control rule requires that a Dynamic HostConfiguration Protocol (DHCP) message received via any one of aplurality of interfaces be processed by a snooping agent, the snoopingagent is configured to update information in the binding table inresponse to processing the DHCP message.
 47. The computer readablestorage medium of claim 46, wherein the access control rule requiresthat the first message be dropped unless the message information matchesthe protocol status information, and the first message is discarded, inresponse to the access control rule.
 48. The computer readable storagemedium of claim 46, wherein the program instructions are furtherexecutable to: update an entry in the access control list to storeinformation encoding the access control rule, wherein the updating theentry in the access control list is performed in response to detectingthat the binding table entry has been updated, and the access controlrule requires that the first message be dropped unless the messageinformation comprises a IP source address that matches the IP address ofthe client identified in the protocol status information.
 49. Thecomputer readable storage medium of claim 46, wherein the binding tableentry is updated in response to detecting a first protocol message, andthe protocol server comprises a DHCP server.
 50. The method of claim 1,wherein the first message is received on an arrival interface that doesnot match the interface identified in the protocol status information, asecond access control rule stored in the access control list requiresthat the first message be discarded unless the first message is receivedon the interface that is identified in the protocol status information,and the first message is discarded, in response to the second accesscontrol rule the message information indicates that the first messagewas sent by a client via a first virtual local area network (VLAN), andat least one protocol message of the one or more protocol messages issent from the protocol server to the client via a second VLAN.
 51. Themethod of claim 1, wherein the message information indicates that thefirst message was sent by a client via a first virtual local areanetwork (VLAN), and at least one protocol message of the one or moreprotocol messages is sent from the protocol server to the client via asecond VLAN.
 52. The method of claim 1, further comprising: performing asecond security action, wherein the access control rule indicatesperformance of the second security action, in response to adetermination that the first protocol message does not correspond to theoutstanding protocol request, and the performing the second securityaction comprises dropping the first protocol message.
 53. The method ofclaim 1, further comprising: performing a second security action,wherein the access control rule indicates performance of the secondsecurity action, in response to a determination that the first messageis sent from the protocol server, the first message is received via afirst interface, and the first interface is not coupled to the protocolserver, and the performing the second security action comprises droppingthe first message.
 54. The method of claim 1, further comprising:performing a second security action, wherein the access control ruleindicates performance of the second security action, in response to adetermination that the first message is sent from a second client, thefirst message is received via a first interface, the first interface isnot coupled to the second client, and the performing the second securityaction comprises dropping the first message.
 55. The method of claim 1,further comprising: performing a second security action, wherein theaccess control rule indicates performance of the second security action,in response to a determination that the first message comprises relayinformation, the relay information is correct, the first message will beoutput to a next network device, and the performing the second securityaction comprises modifying the relay information.
 56. The method ofclaim 1, further comprising: performing a second security action,wherein the access control rule indicates performance of the secondsecurity action, in response to a determination that the first messagecomprises relay information, the relay information is correct, the firstmessage will be output to a client, and the performing the secondsecurity action comprises removing the relay information.